The present invention relates to an electric discharge machining apparatus using a wire electrode as a tool for supplying electric discharge energy between the wire electrode and an electrically conductive subject to be machined to machine the subject into a desired shape. Particularly the invention relates to improvement in a sealing unit, which suppresses outflow of a machining solution from a machining tank when the subject is machined while immersed in the machining solution.
FIG. 8 is a schematic side view showing a conventional wire electric discharge machining apparatus. In the drawing, the reference numeral 1 designates a machining tank disposed on an X-Y cross table of an apparatus body not shown. A table 3 is disposed in the inside of the machining tank 1. The machining tank 1 is set in a state in which a subject 6 to be machined is immersed in a machining solution 7. Reference numeral 10 designates an upper wire guide device. A wire guide, which supports a wire electrode 11 so that the wire electrode 11 can slide freely, is provided in the inside of the upper wire guide device 10. The upper wire guide device 10 is retained above the subject 6 by a Z axis 13 which moves up and down in the drawing. Reference numeral 15 designates a lower wire guide device. A wire guide, which supports the wire electrode 11 so that the wire electrode 11 can slide freely, is provided in the inside of the lower wire guide device 15. The lower wire guide device 15 is retained in a position below the subject 6 and opposite to the upper wire guide device 10 by a lower arm 17. Reference numeral 20 designates a long hole which is provided in a side surface of the machining tank 1 and which is long in a front-and-rear direction of the paper plane of the drawing. The lower arm 17 passes through the long hole 20. The long hole 20 has an opening size which allows the X-Y cross table to move. Reference numeral 24 designates a sealing unit, which is disposed so as to cover the long hole 20 formed in the machining tank 1. The sealing unit 24 has: a base 23 fixed to the machining tank 1 and having a long hole 23h like the long hole 20 of the machining tank 1; a sealing plate 25 defining hole 25h allowing the lower arm 17 to pass therethrough and having a length sufficient to cover the machining tank 1 and the long hole 23h of the base 23; and a first packing 26 attached to the base 23 in order to minimize leakage of the machining solution 7 from between the base 23 and the sealing plate 25. Reference numeral 27 designates a roller retaining mechanism. The roller retaining mechanism 27 has a shaft 28, and a bearing 29. The roller retaining mechanism 27 is formed so that the bearing 29 can rotate freely relative to the shaft 28. The roller retaining mechanism 27 is fixed to the base 23 so that the sealing plate 25 is retained so as to be movable in a front-and-rear direction of the paper plane of the drawing. Reference numeral 30 designates a second packing which is fixed to the sealing plate 25 in order to prevent the machining solution 7 from leaking from a gap between an outer circumferential portion of the lower arm 17 and the through-hole 25h of the sealing plate 25.
An operation of the conventional apparatus configured as described above will be described below. At the time of machining, while a voltage supplied from a power supply not shown is applied between the wire electrode 11 and the subject 6 to generate electric discharge therebetween, the subject 6 is moved with respect to the wire electrode 11 by the X-Y cross table not shown to machine the subject 6. On this occasion, the subject 6 is entirely immersed in the machining solution 7 an upper surface thereof, stability of electric discharge is obtained so that the wire electrode 11 becomes hard to break. Therefore, the machining solution 7 is stored in the machining tank 1. In this case, leakage of the machining solution 7 from the long holes 20 and 23h, which are formed in the machining tank 1 and the base 23, respectively and are pierced by the lower arm 17 is suppressed by entirely covering with the sealing plate 25. That is, when the X-Y cross table moves, the three of the machining tank 1, the base 23 and the first packing 26 move relative to the sealing plate 25. When the machining tank 1 moves in a horizontal direction (in the front-and-rear direction of the paper plane of FIG. 8) with respect to the lower arm 17, the distance between the sealing plate 25 and the base 23 is kept constant by the roller retaining mechanism 27 so that the sealing plate 25 and the base 23 are retained in positions where they always come into contact with the first packing 26. In this state, while the sealing plate 25 and the first packing 26 slide on each other, the machining tank 1 moves in the horizontal direction. On the other hand, when the machining tank 1 moves in an axial direction (in a left-and-right direction of the paper plane of FIG. 8) of the lower arm 17, the machining tank 1 moves while the lower arm 17 and the second packing 30 slide on each other. Hence, leakage of the machining solution 7 from the gap between the base 23 and the sealing plate 25 is suppressed by the first packing 26. Further, leakage of the machining solution 7 from the gap between the outer circumferential portion of the lower arm 17 and the through-hole 25h of the sealing plate 25 is suppressed by the second packing 30.
Since the conventional sealing unit in the electric discharge machining apparatus is configured as described above, the machining solution 7 in the machining tank 1 is contaminated with foreign matter such as sludge at the time of machining. If such a state continues for a long time, the foreign matter is stuck and deposited on the machining tank 1 side surface of the sealing plate 25. For this reason, the coefficient of friction between the first packing 26 and the sealing plate 25 increases, and the resistance to sliding of the sealing plate 25 also increases. When the resistance to sliding of the sealing plate 25 increases, load imposed on the lower arm 17 increases. As a result, the lower arm 17 is deformed to reduce machining accuracy as shown in FIG. 9, which is a plan typical view slightly exaggerated.
Moreover, when the foreign matter such as sludge increases, a gap is produced between the first packing 26 and the sealing plate 25. As a result, the quantity of leakage of the machining solution 7 increases. Hence, there may arise a problem that the machining solution 7 cannot be stored in the machining tank 1.
The present invention is designed to solve the aforementioned problems in the background art and an object of the invention is to provide a wire electric discharge machining apparatus in which foreign matter such as sludge produced at a time of machining is not deposited on a surface of a sealing plate and the foreign matter can be removed even if the foreign matter is deposed on the surface of the sealing plate.
In the wire electric discharge machining apparatus according to the invention, a sealing unit is constituted by a sealing plate and a packing, the sealing plate for blocking long hole of a machining tank, a groove is formed along an outer periphery of the long hole, and nozzles are provided for jetting the machining solution into the groove, and a machining solution jetting unit is provided for jetting the machining solution from the nozzles into a small gap formed between a peripheral portion of the long hole and the sealing plate. Further, in the wire electric discharge machining apparatus according to the invention, a plurality of the nozzles are provided and apertures of low-pressure nozzles are made larger than those of high-pressure nozzles. Further, the wire electric discharge machining apparatus according to the invention further has a controller for discriminating between at least two machining states of rough machining and final machining, and a unit for changing the flow rate of the machining solution jetting by the machining solution jetting unit by using a plurality of circuits different in flow rate of the machining solution fed, wherein the flow rate of the machining solution jetted is controlled to be small at a time of rough machining and large at a time of final machining. Further, in the wire electric discharge machining apparatus according to the invention, a filtration unit for the machining solution jetted from the machining solution jetting unit is provided in the circuit of the machining solution.